This invention relates to subsurface pumps for pumping fluids from boreholes and, more particularly, to pumps for pumping fluids with a high gas content from boreholes.
A subsurface pump is a positive displacement type pump. It consists of a cylindrical barrel in which a hollow plunger, standing valve (inlet), and traveling valve (exhaust) act to raise the fluid from below ground to the surface. The force necessary to move the plunger within the barrel is transferred from the surface pumping unit through a string of sucker rods to the pump which is set at a predetermined location in the hole. At the end of a down stroke of the pump both valves are closed. On the up stroke, the weight of the fluid in the plunger annulus keeps the traveling valve closed. Differential pressure created by the upward movement of the closely fitted plunger and the pressure of the fluid below the ball forces the standing valve open. This allows fluid to flow into the space in the barrel previously occupied by the plunger. On the down stroke, the ball drops into place to close the standing valve. As the hollow plunger passes through the fluid trapped in the barrel, the pressures above and below the plunger are equalized. At this point, assuming that the barrel contains non-compressible liquid rather than compressible gas, the traveling valve is forced open. On the other hand, if the barrel contains primarily gas, the gas compresses and does not develop sufficient force to open the traveling valve. Upon opening of the traveling valve, the fluid flows through the open-top traveling valve cage into the upper part of the barrel and into the tubing. On subsequent up strokes, the fluid column is displaced into the tubing. Each stroke raises the fluid level in the tubing. On further down strokes, more fluid is forced through the plunger into the barrel and tubing. Eventually the fluid in the well is displaced at the surface. However, in gaseous wells, the chambers of a tubing pump will not allow sufficient compression to control gas-related problems and heretofore tubing pumps have not been recommended for medium to extreme gaseous wells.
Pumps are conventionally placed down boreholes to pump fluids, such as water or oil, from boreholes. A typical pump may comprise at least two check valves: a standing ball and seat check valve for admitting fluid above the standing valve and checking the flow of fluid beneath the valve, and a traveling ball and seat check valve for admitting fluid above the valve during a down stroke of the pump and checking the fluid flow during an up stroke of the pump. During the up stroke, a reduced pressure is created above the standing valve to cause the valve to open, admitting fluid above the standing valve and below the traveling valve. During the down stroke, the standing valve closes and fluid pressure beneath the traveling valve normally forces the traveling valve to open so that fluid flows through the traveling valve into the upper pump chamber during the down stroke.
There are, however, many wells which produce fluids having a high gas content. The pumping efficiency of conventional pumps, as hereinabove discussed, is considerably reduced, and pumping action can be completely blocked. While a liquid is substantially incompressible, hydraulically opening the check valves during the reciprocating pump stroke, a gas is compressible. Thus, gas located between the traveling check valve and the standing check valve can merely compress during the down stroke without generating sufficient pressure to open the traveling valve. No liquid is then admitted above the valve to be lifted during the up stroke and the pump is gas locked. This problem is aggravated in large bore pumps, where considerably more internal volume is available for gas accumulation, with concomitant low pressurization during compression.
There have been attempts to alleviate the problem of gas locking. U.S. Pat. No. 3,941,516, issued Mar. 2, 1976, to Soberg, provides a head valve above the traveling valve to relieve pressure above the traveling valve during the down stroke to minimize the pressure needed to unseat the valve. U.S. Pat. No. 4,557,668, issued Dec. 10, to Jones, also includes a head valve (gas release valve therein). Jones further includes a trip to release the head valve at the upper end of the stroke to release fluid being carried upward by the traveling valve.
Each of the above pumps requires differential pressure to unseat the traveling check valve, and therefore that pressure equalization and gas release is not assured. Also, the pumps can act to remove all liquids from above the standing valve and there is no remaining fluid for the equalizer to unseat the traveling valve at the lowest part of the down stroke. Further, none of the above pumps would be suitable for use as an enlarged bore pump since the traveling valve assemblies are rotatable within the traveling cage and the sucker rod cannot be disengaged.
A stroke-through pump presents a particular problem for an equalizing assembly. In such a pump the travelling valve and equalizer assembly pass from a relatively large diameter portion of the pump barrel into a relatively smaller diameter portion of the pump barrel having a close tolerance between the inside diameter of the pump barrel and outside diameter of the pump plunger and equalizer assembly. Such a transition is necessary in a stroke-through pump to clean particulate matter from the pump on the upstroke. However, the traveling valve and equalizer assembly must remain in alignment and have sufficient lateral stability to prevent the equalizer assembly from hanging up on that portion of the barrel where it reduces in diameter. Absent such alignment and stability, the equalizer assembly is likely to be damaged or snapped off on the upstroke.
These problems, and others, are addressed by the present invention, wherein a pressure equalizer assembly is provided for positively unseating a traveling check valve at end portions of the reciprocating stroke while enabling the check valve to seat normally for fluid pumping.
Accordingly, it is an object of the present invention to provide for mechanically unseating the traveling check valve at a location nearest the standing check valve.
It is another object of the present invention to provide a traveling check valve unseating mechanism for reciprocating with the traveling valve that maintains its alignment relative to the pump and is locked against rotation relative to the traveling valve cage.
Still another object is to enable some liquid to escape from the pump chamber downwardly through the traveling check valve cage to provide a non-compressible fluid at the bottom of the down stroke to activate the equalizing device.
Another object is to enable the pump to be separated from the sucker rods in the borehole or larger-oversized pumps when the pump plunger diameter is larger than the tubing diameter.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.